A boat on the Hudson River with the Lower Manhattan skyline in the background on a cloudy Thursday morning.Credit Julio Cortez/Associated Press

Hurricane Joaquin has forced the Republican presidential candidates Donald J. Trump and Gov. Chris Christie to cancel almost all campaign events in the coming days, with Mr. Christie declaring a state of emergency in New Jersey. Mr. Christie still plans to travel to Boston for a scheduled event on Thursday night, but will fly back to his home state shortly afterward.

“Many of us have been through this before, with Irene and with Sandy,” Mr. Christie said at a morning news conference in Trenton.

The storm, a Category 4 hurricane barreling north in the Atlantic Ocean, is proving hard for forecasters to predict. Southern New Jersey, which is still healing the wounds of Hurricane Sandy in 2012, is squarely in the “cone of unpredictability.”

So Mr. Christie is taking no chances with this storm, canceling his events in New Hampshire and promising to be in New Jersey until Tuesday, at least.

Mr. Trump’s campaign said that he is canceling a Friday night rally in Virginia Beach, which is also in the path of the storm. Mr. Trump said in a statement: “While we were expecting 10,000 people based on demand, and it would have been a happening, everyone’s safety is of far more importance to me. We will reschedule the event very soon and I look forward to being there.”

In New Jersey, Mr. Christie was a constant presence during Hurricane Sandy and its immediate aftermath, his blue governor’s fleece seen up and down the Jersey Shore for weeks. He also took some political criticism from Republicans for embracing President Obama just a few days before the 2012 election.

But those political memories were far from Mr. Christie’s mind on Thursday as he detailed the state’s preparedness for the storm, most of it told through the prism of Hurricane Sandy. He repeatedly told the residents of New Jersey that, despite the memories of the devastating storm three years ago, to remain calm.

“I’m much better prepared, the lieutenant governor is better prepared, and for all the people up here who lived through Sandy, they’re much better prepared,” Mr. Christie said.

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